A circuit switched path is created when two devices or entities in a network establish a dedicated communications path that allows the two entities to communicate directly with each other. This dedicated and reserved communications path is typically established before the two entities start communicating. Early analog telephone systems use such circuit switching. Traditional methods of voice calling via mobile networks also use circuit switching. Circuit switching is still used in some networks for voice calls, particularly in legacy networks.
Packet switching is different from circuit switching and occurs when two devices or entities in a network do not establish a dedicated communications path in advance, but instead the data to be communicated from one entity to the other is partitioned into packets, and each packet is forwarded from the sender to the receiver, perhaps travelling along a different path in the network compared to the other packets. The network architecture of the Evolved Packet System (EPS) has been designed to support packet switching only, not circuit switching.
It may be the case that a call is initiated and set up in a packet switched (PS) domain, such that the call is ongoing in a PS session. Then, during the call, it is desired to transfer the call to a circuit switched (CS) session in the CS domain. Please consider the following example. Assume the call is initiated in the EPS and is anchored in the IP Multimedia Subsystem (IMS). The call is therefore initially ongoing in a PS session in the PS domain. Then, while the call is ongoing, conditions of the EPS network deteriorate (e.g. the mobile device begins moving out of the range of all evolved Node Bs in the EPS network). However, it is determined on the network side that a CS connection can be established and offered through a neighbouring Node B (e.g. via a Universal Terrestrial Radio Access Network or a GSM EDGE Radio Access Network). The circuit is set up and the mobile device is handed over to neighbouring Node B, and the call continues, but in a CS session in the CS domain.
When moving an ongoing call from a PS session to a CS session, such as in the scenario above, it is desired to reduce or avoid disruption of the call. More generally, when moving a voice call from one communication session to another communication session (e.g. from one PS session to another PS session, or from a PS session to a CS session, or from a CS session to a PS session, or from one CS session to another CS session), it is desired to reduce or avoid disruption of the call.
Like reference numerals are used in different figures to denote similar elements.